Interestingly, last summer, my eldest adult son and I were shooting and he complained of the same scenario. He couldn't see the crosshairs on his standard duplex reticle. We tried adjusting the ocular lens, to no avail. Screwed it in and out and he complained that when he looked through it at the blue sky, he could see the center crosshair of the duplex reticle, but when he looked through it at the target, the center crosshair simply "washed-out".

I focused the scope to my eyes. No problem. I can see through it just fine. So, we started digging around in my boxes and found a scope he could use. Mounted it on the rifle and adjusted the focus to his eye, he can see and shoots it just fine.

It was something about that scope and his eye, and he couldn't use it. That scope is now mounted on another rifle, giving fine service. Go figure.